Field of the Invention
The invention relates, in general to a method for producing a light-emitting component, and in particular, to a method for producing a light-emitting component, in which, after the formation of a layer sequence including at least one active layer on the front side of a semiconductor base substrate, the base substrate is at least partially removed and the layer sequence is subsequently joined to a foreign substrate.
Light-emitting semiconductor components are produced on the basis of III-V semiconductor systems in which a light-emitting layer sequence including an active layer is deposited epitaxially on a semiconductor base substrate made of GaAs, for example. The active layer is, for example, InGaAlP with different Al concentrations. For diverse applications, it is necessary that the epitaxially applied layer sequence be removed from the base substrate (so-called epitaxial lift-off) and be fixed on a different carrier (foreign substrate) with the formation of good electrical contacts. Depending on the desired application and the fabrication technology used, the problems that have to be solved when producing individual components are different from the problems that have to be solved when producing monolithic integrated circuits. A significant part of the visible light emitted by the semiconductor chip is absorbed by the base substrate (for example GaAs), as a result of which the external efficiency of light generation becomes minimal. A substrate (for example GaP) which is transparent to the emitted light is thus preferred in order to achieve extremely high light intensities typically of 5 lm or more, and efficiencies typically of more than 10%. At the same time, there is a desire to provide good electrical coupling between the epitaxial layer sequence and the foreign substrate even when high currents are used (with a minimal total forward voltage of the LED semiconductor chip), and to provide a high yield during production. Typical applications for such individual semiconductor components are exterior lighting, lamps, and the like in motor vehicles. Furthermore, it is possible to realize optoelectronic integrated circuits by implementating extremely small III-V semiconducting epitaxial layers in silicon-based integrated circuits. In this case, the electrical coupling of the III-V semiconductor component to the silicon component is crucial.
Typical examples of applications for these are LED displays, optical information processing systems, and the like.
The fabrication, fixing and electrical contact-making of an epitaxial layer on a foreign substrate have to date been carried out essentially by two methods known as heteroepitaxy and fusion of epitaxial layers on fine substrates.
In the case of the heteroepitaxy for example of InGaAlP on GaP, high dislocation densities are inevitably produced because of the large lattice mismatches of the materials used. The dislocation densities can be reduced either by reducing the epitaxial areas using SiO2 masks, thereby enabling strain to be relieved more easily, or by conventional methods such as, for example, thermally cyclic crystal growth, introduction of interfaces and the like. Nevertheless, the high density of dislocations leads to intensification of non-radiating recombination processes and thus to reduced light emission, as well as to undefined additive voltage drops across the chip, which is unfavorable for the electrical coupling.
In the case of the fusion of epitaxial layers on foreign substrates, the absorbing GaAs base substrate is removed wet-chemically from the epitaxial layer sequence by selective undercutting, an AlN layer having been introduced beforehand. The remaining epitaxial layer sequence is applied to a transparent GaP foreign substrate under high pressure and at high temperature. The epitaxial layer sequence adheres on the transparent foreign substrate by way of the formation of van der Waals bonds.
All of the production methods discussed present the disadvantage of having undefined junctions between the two bodies to be joined, which are essentially caused because of an inhomogeneous etching away of the absorbing substrate over a few hundred micrometers. The inhomogeneous van der Waals bonds between the epitaxial layer sequence and the foreign substrate and the associated formation of oxides can lead to unfavorably high voltage drops across the semiconductor chip and can considerably reduce the yield. Thus, in the case of commercially available LED semiconductor components for high-current applications in which the InGaAlP layer sequence was joined by fusion on a GaP foreign substrate, forward voltages of 2.4 mV and above were measured at 70 mA, which greatly limits their application.
Published European Patent Application EP 0 616 376 A1 discloses a method for wafer bonding of LED layers, wherein the LED layers are produced on a growth substrate which is subsequently removed. The LED layers are joined in a planar manner to a second substrate of suitable optical properties using a conventional wafer bonding technique in order to obtain a low contact resistance or desired optical interface properties with the second substrate.
Published Japanese Patent Application JP-A 5 251 739 discloses a light-emitting semiconductor device which provides for a red GaAsP-LED and a green GaP-LED to be electrically contact-connected in order to produce a mixed color. A eutectic alloy is provided for electrically contact-connecting the two LEDs to one another.